The present invention relates to compositions for use in making inner covers, outer covers, intermediate layers and cores for a golf ball, and it more specifically relates to such golf ball layers composed of post-crosslinkable urethane. The present invention also relates to methods of manufacture of golf balls incorporating these layers.
Golf balls generally include a core and at least one cover layer surrounding the core. Balls can be classified as two-piece, multi layer, or wound balls. Two-piece balls include a spherical inner core and an outer cover layer. Multi-layer balls include a core, a cover layer and one or more intermediate (or mantle) layers. The intermediate layers themselves may include multiple layers. Wound balls include a core, a rubber thread wound under tension around the core to a desired diameter, and a cover layer, typically of balata material.
Generally, two-piece balls provide good durability and ball distance when hit, but they provide poor ball control, due to low spin rate and poor “feel” (the overall sensation transmitted to the golfer while hitting the ball). Wound balls having balata covers generally have high spin rate, leading to good control, and good feel, but they have short distance and poor durability in comparison to two-piece balls. Multi-layer balls generally have performance characteristics between those of two-piece and wound balls. Multi-layer balls exhibit distance and durability inferior to two-piece balls but superior to wound balata balls, and they exhibit feel and spin rate inferior to wound balata balls but superior to two-piece balls.
Material characteristics of the compositions used in the core, cover, and any intermediate layers are important in determining the performance of the resulting golf balls. In particular, the composition of the cover layer is important in determining the ball's durability, scuff resistance, speed, shear resistance, spin rate, feel, and “click” (the sound made when a golf club head strikes the ball). Various materials having different physical properties are used to make cover layers to create a ball having the most desirable performance possible. For example, many modern cover layers are made using soft or hard ionomer resins, elastomeric resins or blends of these. Monomeric resins used generally are ionic copolymers of an olefin and a metal salt of a unsaturated carboxylic acid, or ionomer terpolymers having a co-monomer within its structure. These resins vary in resiliency, flexural modulus, and hardness. Examples of these resins include those marketed under the name SURLYN manufactured by E.I. DuPont de Nemours & Company of Wilmington, Del., and IOTEK manufactured by Exxon Mobil Corporation of Irving, Tex. Elastomeric resins used in golf ball covers include a variety of thermoplastic or thermoset elastomers available. Layers other than cover layers also significantly affect performance of a ball. The composition of an intermediate layer is important in determining the ball's spin rate, speed, and durability. The composition and resulting mechanical properties of the core are important in determining the ball's coefficient of restitution (C.O.R.), which affects ball speed and distance when hit. In addition to the performance factors discussed above, processability also is considered when selecting a formulation for a golf ball composition. Good processability allows for ease of manufacture using a variety of methods known for making golf ball layers, while poor processability can lead to avoidance of use of particular materials, even when those materials provide for good mechanical properties.
Various materials having different physical properties are used to make ball layers to create a ball having the most desirable performance possible. Each of the materials discussed above has particular characteristics that can lead to ball properties when used in a golf ball composition, either for making a ball cover, intermediate layer, or core. However, one material generally cannot optimize all of the important properties of a golf ball layer. Properties such as feel, speed, spin rate, resilience and durability all are important, but improvement of one of these properties by use of a particular material often can lead to worsening of another. For example, ideally, a golf ball cover should have good feel and controllability, without sacrificing ball speed, distance, or durability. Despite the broad use of copolymeric ionomers in golf balls, their use alone in, for example, a ball cover can be unsatisfactory. A cover providing good durability, controllability, and feel would be difficult to make using only a copolymeric ionomer resin having a high flexural modulus, because the resulting cover, while having good distance and durability, also will have poor feel and low spin rate, leading to reduced controllability of the ball. Also, the use of particular elastomeric resins alone can lead to compositions having unsatisfactory properties, such as poor durability and low ball speed.
Therefore, to improve golf ball properties, the materials discussed above can be blended to produce improved ball layers. Prior compositions for golf balls have involved blending high-modulus copolymeric ionomer with, for example, lower-modulus copolymeric ionomer, terpolymeric ionomer, or elastomer. As discussed above, ideally a golf ball cover should provide good feel and controllability, without sacrificing the ball's distance and durability. Therefore, a copolymeric ionomer having a high flexural modulus often is combined in a cover composition with a terpolymeric ionomer or an elastomer having a low flexural modulus. The resulting intermediate-modulus blend possesses a good combination of hardness, spin and durability.
However, even with blending of materials to improve ball properties, use of the materials and blends discussed above has not been completely satisfactory. Improving one characteristic can lead to worsening of another. For example, blending an ionomer having a high flexural modulus with an ionomer having a low flexural modulus can lead to reduced resilience and durability compared to use of the high-modulus ionomer alone. Also, the hardness of the compositions that can be obtained from these blends are limited, because durability and resilience get worse when hardness is lowered by increasing terpolymeric content of these blends. In general, it is difficult to make materials for use in, for example, a golf ball cover layer that possess good feel, high speed, high resilience, and good shear durability, and that are within a wide range of hardness. Additional compositions meeting these criteria are therefore needed.
Conventionally, golf ball cover and intermediate layers are positioned over a core or other internal layer using one of three methods: casting, injection molding, or compression molding. Of the three methods, injection molding is most preferred, due to the efficiencies gained by its use. Injection molding generally involves using a mold having one or more sets of two hemispherical mold sections that mate to form a spherical cavity during the molding process. The pairs of mold sections are configured to define a spherical cavity in their interior when mated. When used to mold an outer cover layer for a golf ball, the mold sections can be configured so that the inner surfaces that mate to form the spherical cavity include protrusions configured to form dimples on the outer surface of the molded cover layer. The mold sections are connected to openings, or gates, evenly distributed near or around the parting line, or point of intersection, of the mold sections through which the material to be molded flows into the cavity. The gates are connected to a runner and a sprue that serve to channel the molding material through the gates. When used to mold a layer onto an existing structure, such as a ball core, the mold includes a number of support pins disposed throughout the mold sections. The support pins are configured to be retractable, moving into and out of the cavity perpendicular to the spherical cavity surface. The support pins maintain the position of the core while the molten material flows through the gates into the cavity between the core and the mold sections. The mold itself may be a cold mold or a heated mold. In the case of a heated mold, thermal energy is applied to the material in the mold so that a chemical reaction may take place in the material. Because thermoset materials have desirable mechanical properties, it would be beneficial to producers of golf balls using this process. Unfortunately, thermoset materials generally are not well suited for injection molding, because as the reactants for thermoset polyurethane are mixed, they begin to cure and become highly viscous while traveling through the sprue and into the runners of the injection mold, leading to injection difficulties. For this reason, thermoset materials typically are formed into a ball layer using a casting process free of any injection molding steps.
In contrast to injection molding, which generally is used to prepare layers from thermoplastic materials, casting often is used to prepare layers from thermoset material (i.e., materials that cure irreversibly). In a casting process, the thermoset material is added directly to the mold sections immediately after it is created. Then, the material is allowed to partially cure to a gelatinous state, so that it will support the weight of a core. Once cured to this state, the core is positioned in one of the mold sections, and the two mold sections are then mated. The material then cures to completion, forming a layer around the core. The timing of the positioning of the core is crucial for forming a layer having uniform thickness. The equipment used for this positioning are costly, because the core must be centered in the material in its gelatinous state, and at least one of the mold sections, after having material positioned therein, must be turned over and positioned onto its corresponding mold section. Casting processes often lead to air pockets and voids in the layer being formed, resulting in a high incidence of rejected golf balls. The cost of rejected balls, complex equipment, and the exacting nature of the process combine to make casting a costly process in relation to injection molding.
Compression molding of a ball layer typically requires the initial step of making half shells by injection molding the layer material into a cold injection mold. The half shells then are positioned in a compression mold around a ball core, whereupon heat and pressure are used to mold the half shells into a complete layer over the core. Compression molding also can be used as a curing step after injection molding. In such a process, an outer layer of thermally curable material is injection molded around a core in a cold mold. After the material solidifies, the ball is removed and placed into a mold, in which heat and pressure are applied to the ball to induce curing in the outer layer.
One material used in ball layers is polyurethane. Polyurethane typically is formed as the reaction product of a diol or polyol, along with an isocyanate. The reaction also can incorporate a chain extender configured to harden the polyurethane formed by the reaction. Thermoplastic polyurethanes have generally linear molecular structures and incorporate physical crosslinking that can be reversibly broken at elevated temperatures. As a result, thermoplastic polyurethanes can be made to flow readily, as is required for injection molding processes. In contrast, thermoset polyurethanes have generally networked structure that incorporate irreversible chemical crosslinking. As a result, thermoset polyurethanes do not flow freely, even when heated.
Thermoplastic and thermoset polyurethanes both have been used in golf ball layers, and each provides for certain advantages. Because of their excellent flowability, thermoplastic polyurethanes can be positioned readily around a golf ball core using injection molding. Unfortunately, golf ball covers comprising thermoplastic polyurethane exhibit poor shear-cut resistance. Thus, while thermoplastic polyurethane covers are less expensive to make due to their superior processability, they are not favored due to the resulting inferior ball performance. In contrast, thermoset polyurethane exhibits high shear-cut resistance and is much more scuff- and cut-resistant than thermoplastic polyurethane. However, the irreversible crosslinks in the thermoset polyurethane structure make it unsuitable for use in injection molding processes conventionally used for thermoplastic materials.
Despite their drawbacks, thermoplastic polyurethanes are used in golf ball compositions. U.S. Pat. No. 5,759,676 to Wu discloses thermoplastic polyurethane utilized in blends for mantle and cover layers. U.S. Pat. No. 6,319,152 to Takesue teaches blending of a thermoplastic polyurethane with a styrene-based block copolymer to increase the scuff resistance of the resulting golf ball cover. The patent discloses that because thermoplastic polyurethanes are “inexpensive and easy to mold, these elastomers are regarded as an excellent cover stock substitute for balata material. However, the thermoplastic polyurethane elastomers are still insufficient in scuff resistance upon iron shots.” Thermoplastic polyurethanes also are used for making mantle layers to give the feel of a wound ball to non-wound constructions. Such a mantle is disclosed in U.S. Pat. No. 5,759,676 to Cavallaro et al.
Though they are more expensive to process than thermoplastic polyurethanes, thermoset polyurethanes also have been used in golf ball layers. For instance, U.S. Pat. No. 6,132,324 to Hubert discloses a golf ball having a cover formed from thermoset polyurethane. The patent teaches a method for casting a thermoset polyurethane cover over an ionomer inner layer, including a step of measuring the viscosity “over time, so that the subsequent steps of filling each mold half, introducing the core into one half and closing the mold can be properly timed for accomplishing centering of the core cover halves fusion and overall uniformity.” The additional steps involved in casting a layer over those needed for injection molding the layer lead to added complexity and expense. Another patent discussing use of thermoset polyurethane is U.S. Pat. No. 6,435,987 to Dewanjee. This patent teaches thermosetting polyurethane comprising a toluene diisocyanate-based prepolymer, a second diisocyanate prepolymer, and a curing agent. Again, this method makes use of casting because the materials used would not be well suited to injection molding.
One method for injection molding of a polyurethane is by using a post-crosslinkable thermoplastic polyurethane. This is a thermoplastic polyurethane which, upon irradiation, is capable of crosslinking to form a thermoset polyurethane. U.S. Pat. No. 6,369,125 to Nesbitt discloses such an approach, whereby a thermoplastic polyurethane having unsaturated carbon-carbon bonds is exposed to electromagnetic radiation to induce crosslinking and form thermosetting polyurethane. The process used in this patent utilizes a co-agent such as a hydroxyl terminated polybutadiene, which enables thermoplastic polyurethane to crosslink upon exposure by radiation. However, the use of radiation is undesirable because its depth of exposure cannot be controlled. As a result, the radiation will travel through the entire ball, affecting parts of the ball other than the layer being molded. For example, certain elastomeric materials used as thread in wound layers are susceptible to degradation from radiation. Once degraded, the thread may snap when the golf ball is struck by a golf club. Radiation also can cause additional crosslinking in the core, ultimately producing a core that is harder than desired, resulting in a degradation of ball performance. Besides the effect on other parts of the ball, radiation also can adversely affect materials blended with the polyurethane to increase or decrease certain ball control properties, such as distance and spin. Finally, radiation causes thermoplastic polyurethane to turn from white to yellow. Though this may be avoided by using antioxidants, these antioxidants may retard crosslinking and thereby frustrate the purpose of irradiation. For these reasons, irradiation of thermoplastic polyurethane to cause crosslinking is not preferred.
In view of the above, it is apparent that polyurethane golf balls that provide the optimal ball performance properties of a thermoset polyurethane, while retaining the superior processability of a thermoplastic polyurethane, as well as methods for making these balls, are needed. The present invention fulfills this need and provides further related advantages.